Power generating electronic and optical devices/systems, as well as energy conversion and storage systems such as fuel cells and the like, have become increasingly compact. At stake has been and is thermal energy management performance, and thus, the reliability of such devices and systems. Increasingly and currently, it is generally believed that improved thermal energy management is requisite to advance such high performance electronic, optical and energy conversion/storage devices and/or systems.
Heat pipes, thermal ground planes and vapor chambers (a/k/a flat heat pipes) are frequently employed for the above noted thermal energy management. Characteristically, such devices combine principles of both thermal conductivity and phase transition to effectively manage a transfer of heat between two solid interfaces. More particularly, such passive heat transfer devices utilize capillary forces to circulate a working fluid between discrete evaporator and condenser regions of a vacuum tight housing, compartment, shell or vessel.
With regard to such devices, a working fluid occupies a vapor chamber having evaporating and condensing portions. In heat spreading applications, heat enters at a select surface location (i.e., area) and exits across the rest of the surfaces. In heat transport applications, a discrete condenser (e.g., a water cooled block) aids heat transport. The evaporating surface is intended to engage/contact a heat source to absorb the heat of the heat source, thereby heating and evaporating the working fluid in the vapor chamber. When the vapor is brought into contact with a “cold” surface (i.e., the condensing surface), the vapor condenses into liquid to release the latent heat of the working fluid. With the phase change between vapor and liquid of the working fluid, the heat of the heat source can be conducted to the condensing surface.
Heat pipes are generally inexpensive, reliable, and long lived, thus they are in wide use. Moreover, they provide effective heat removal over long distances and can be effective for applications characterized by high g-forces, shock, vibration, and freeze/thaw. Further still, their use enables smaller and more compact arrangements of electronics as is characteristic of hand held devices and avionics. Present technical efforts include those directed to flexible and conformal thermal grounds planes, more particularly, to ease of manufacture and reliability thereof.
Rosenfeld et al. (U.S. Pat. No. 6,446,706), and later Kim et al. (U.S. Pub. No. 2008/0210407 A1) citing same, both incorporated herein by reference in their entireties, generally express concerns relating to the exacerbated tensions of maintaining uniform lamina contacts in furtherance of predictable, repeatable and reliable heat transfer, and establishing suitable capillary force/wicking, to facilitate phase change of a working fluid, in the context of thin, flat panel type heat transfer devices. As to the former, each outer heat pipe wall of the purportedly especially flexible heat pipe is characterized by a plurality of lamina, among others, two/dual metal foil layers intended to function as barriers to material ingress/egress. As to the later, in furtherance of an aim of ensuring high thermal conductivity at a low cost, a panel type heat transfer device characterized by one or more aggregated hydrophillic fiber wick structures and a directional coolant passage is provided. Be that as it may, manufacturing ease for such devices has remained elusive, with reliability wanting.
Issues of low production yield, low vapor-liquid circulation efficiency, and poor internal supporting strength remain as indicated by Yang et al. (U.S. Pat. No. 8,997,839) and Yang (U.S. Pat. No. 9,021,698), each incorporated herein by reference in their entireties. The former teaching provides a thin sheet element characterized by intersectingly extending sections which define element openings, and bosses fixedly located in select openings thereof, the bosses and thin sheet member being disposed in a receiving space of a pipe body at the same time. The latter teaching provides a flat plate heat pipe characterized by only two soldered sealing joints (i.e., the pipe is flattened and the two free ends sealed) and a sintered supporting layer which functions to avoid plate deformation while apertures thereof permit passage of a phase-change media therethrough and there across.
Beyond the aforementioned developments and perceived shortcomings, capillary wick structures have deservingly been a focal point for thermal ground plane advancement. With reliance upon material science advances, improved thermal conductivity and coefficient of thermal expansion matching have been generally realized in and for capillary wick structures, with working in this area ongoing. Conventional metal powder based wicking structures (e.g., sintered copper) have generally been succeeded, for instance and without limitation, by those characterized by copper foam, copper micro and nano structures (and hybrids thereof), with and without a hydrophilic coating (e.g., an atomic layer deposited (ALD) film), diamond-copper composites, titanium and titania micro and nano structures, and carbon micro and nano structures.
Be that as it may, it is well known and appreciated that a hermetic seal of the thermal ground plane structure is considered critical. Generally known and practiced techniques or methods include, for example, solder bonding, polymer bonding, vacuum brazing, and electron beam welding. Moreover, in addition to a combined bonding approach such as one characterized by a fluorinated ethylene propylene (FEP), e.g., Teflon®, bond with a soldered perimeter or periphery, metal plating over an FEP bonded structure or element, as by ALD, is likewise known and practiced.
While a variety of sealing techniques are known, it is believed advantageous and desirable to overcome known shortcomings. For instance, polymer bonded device peripheries are known to be gas permeable and of suspect reliability for the long haul, with solder bonded device peripheries, by their nature, introducing a potential deleterious solder alloy and/or solder flux contamination interior of the seal (i.e., reacting with the working medium within the chamber), with such union providing/imparting no mechanical strength. Moreover, metal plating over a bonded device periphery to form or fortify the hermetic seal may not be feasible, and contrariwise, it likewise introduces a potential deleterious plating fluid contamination interior of the seal (i.e., reacting with the working medium within the chamber). Finally, while more reliable hermetic seals are achievable, the relative high cost prevents their ubiquitous application.
In addition to the heretofore cited thermal management devices, devices characterized by phase change material (PCM) are likewise known and generally valued for their energy absorption, heat storage and/or heat dissipation functionality and/or characteristics. PCMs, which are characterized by a high heat of fusion, are capable of storing and releasing large amounts of energy. Heat is absorbed or released when the material changes from solid to liquid and vice versa, with PCMs thusly classified as latent heat storage (LHS) units. While latent heat storage can be achieved through solid→liquid, solid→gas and liquid→gas phase changes, only solid→liquid and liquid→solid phase changes are practical for PCMs.
With numerous electronic devices, especially phones, hand helds, tablets, etc. operating at fluctuating power levels (i.e., periods of low and high power demands), the attenuation or buffering of variable thermal demands/loads via a heat sink (e.g., a PCM heat sink) is believed desirable and advantageous owing to a promise of improved electronic device performance and reliability.
That said, PCM have their limitations and/or shortcomings, most notable, heat distribution. While known PCMs have undergone adaptation to improve or enhance energy absorption, heat storage and/or heat dissipation characteristics for passive thermal control (see e.g., Outlast® LHS heat spreaders from Outlast Technologies LLC, Colorado, USA), efforts in this area nonetheless continue.
In light of the foregoing, there thus remains a need to provided a low cost, long lived thermal ground plane characterized by a highly reliable hermetic seal. Moreover, there thus remains a need to provided a low cost, long lived thermal ground plane having structural chamber members formable within a device periphery characterized by a highly reliable unions of select opposing chamber portions. Further still, it is believed desirable and advantageous to provide an elegant, low cost process for fabricating high reliability thermal ground planes. Yet further and finally, it is believed desirable and advantageous to improve PCM performance via an adaptation wherein an improved thermal ground plane is operatively combined with a PCM, adapted or otherwise.